Me Vs. Estoy
If some asks me "cómo estás?" should i reply
Me alegra.
or....
Estoy alegra.
3 Answers
Even though your question was about the adjective alegre, you can use the verb alegrar to express yourself also.
me alegra is expressing that something is making me happy, so you can't just say me alegra, you have to tell what it is. This situation or whatever is happening is having an effect on you and that is what you are expressing.
Me alegra de que vayas a traer un pastel a la fiesta. It makes me happy that you're bring a cake to the party (hopefully).
So, it can't be a response to "how're you doing?"
When you want to put the emphasis on yourself, you can say (yo) me alegro de que (notice the conjugation) is expressing my feeling, I am happy because of something.
Me alegro is expressing my emotion, me alegra is expressing an effect of something on me (happiness).
Me alegra.
That sentence is correct, but it is incomplete, since you are literally saying "[It] makes me happy...", without saying what makes you happy. You can use that phrase in sentences like these:
Me alegra que estés aquí = I am happy that you are here
it should be "Estoy alegre" or "Estoy contenta"